1. Field of the Invention
The present invention relates to a system for controlling a network activation sequence in an integrated service digital network (ISDN), more particularly to the system for controlling the network activation sequence between a network terminal and an exchange terminal.
2. Description of the Related Art
In future information and communication systems, a greater variety of information will be handled--including voice, data, picture, and other information. To deal with this increased variety of information, a network will have to be able to integrate as many compound telecommunication services as possible. Further, common use of transmission exchange equipment, etc. for many kinds of communication services will have to be considered. Therefore, research is being conducted for the realization of an integrated network combining a transmission system and exchange system by digital technology.
In a standard construction of an ISDN subscriber line in an integrated network, terminal equipment such as a telephone or a data terminal is connected via a network terminal to an exchange terminal.
In the network, the transmission line between the terminal equipment and network termination and the transmission line between the network termination and exchange terminal, known as the "physical layer", provide the physical transmission lines between the network side and the user side.
The transmission lines are placed in a deactivated state when idle, i.e., when awaiting use. This is basically to decrease power consumption as much as possible. Therefore, to enable actual communication between the network side and the user side, the transmission lines must be switched from the deactivated state to the activated state.
The International Consultative Committee for Telephone and Telegraph (CCITT) is conducting studies with the eventual aim of prescribing the S/T point in this standard construction, that is, the signal sequences of only private subscriber lines between the terminal equipment and network terminal. It does not intend to prescribe the U point, that is, between the network terminal and the exchange equipment. With respect to the activation sequences at the S/T point, the CCITT is moving in the direction of issuing some specific recommendations, as mentioned later.
Further, in the network mentioned above, a two-wire time-division transmission system (so called as "ping-pong" transmission system) is used for the digital subscriber transmission system. In a two-wire time-division transmission system, two wires are used to connect the network terminal and the exchange terminal, enabling alternate transmission and reception of digital signals in a burst manner.
Network terminal apparatuses are provided in the exchange terminal and in the network terminal respectively. The receiving level of a signal receiving equalizer provided in the network terminal apparatus, is set before the transmission of information, so that the exchange terminal side and the network terminal side are set to the most suitable receiving level state. This is done by a training pattern sent between the network terminal side and the exchange terminal side. When setting the receiving level of the equalizer on the network terminal side, a training pattern is usually sent from the exchange terminal side when it is not in the communication state. The network terminal side network terminal apparatus receives this training pattern to set the receiving level of the equalizer. The network terminal side is thus continuously held in the synchronized state.
In this case, the network terminal side network terminal apparatus does not send out any signal at all to the transmission line. On the other hand, when the network terminal side network terminal apparatus is activated from the exchange terminal side or it is activated from the network terminal side, it sends out the training pattern, by which the receiving equalizer in the exchange terminal side network terminal apparatus sets the receiving level. When the receiving level of each equalizer is set in the exchange terminal side and the network terminal side, the exchange terminal side and the network terminal side become synchronized and communication is carried out.
Now, when the receiving level of the equalizer is set in the network terminal side network terminal apparatus by the training pattern from the exchange terminal side, there is no input signal from the transmission line in the exchange terminal side receiving equalizer, so setting of the receiving level is not carried out. However, in actuality, an input signal is supplied to the receiving equalizer of the exchange terminal side network apparatus due to various causes. That is, in the ping-pong transmission system, the transmission line is used by time division, and transmission and the reception are switched with burst periods. At this time, noise generates at the time of switching between transmission and reception. Further, the training pattern leaks via a transformer, which connects the sending side and the receiving side in the network terminal apparatus to the line. Thus, the receiving level of the exchange terminal side receiving equalizer is undesirably set.
The levels of these signals are usually high, so, if the receiving level of the equalizer in the exchange terminal side is set by these signals, when synchronization is established in the exchange terminal side by the start signal from the network terminal and the training pattern, the problem arises that the exchange terminal side cannot receive the signal attenuated by the transmission line from the terminal side.
For the purpose of solving this problem, conventionally, the network terminal side network terminal apparatus is powered from the exchange terminal side, and the network terminal side forms a direct current loop before the start for the exchange terminal side. The exchange terminal side sets the receiving equalizer in the initial state by detecting this loop and receives the training pattern next sent from the network terminal, so that the receiving level of the receiving equalizer in the exchange terminal side network terminal apparatus is set.
However, such a system presupposes that the power for the network terminal side is supplied from the exchange terminal side. When the power of the network terminal side network terminal apparatus is supplied between apparatuses, the above mentioned direct current loop does not exist, so that the initial setting of the exchange terminal side receiving equalizer cannot be made. Therefore, the receiving level of the exchange terminal side receiving equalizer cannot be set correctly by the training pattern from the network terminal.